YidnaSkidna
Civil/Environmental
Is anyone aware of a standard pavement section for a school bus parking lot? Assume stable soils and geotextile underlayment. Thanks.
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Bus Parking Lot X-Section
- Thread starter YidnaSkidna
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There is never a "standard" flex pave section. It must be calculated to yield the SN required for ESALs.
A "typical" one, assuming well-drained, well-consolidated/compacted soils would be:
no geotextile (why are you using that with good soils?),
6-8" Course Agg. of #200 - 2", D50= 3/4" (PennDOT calls it "2A"),
2.5" BCBC (PennDOT calls it "ID-2"),
1.5" ID-2 Wear Course.
Engineering is the practice of the art of science - Steve
A "typical" one, assuming well-drained, well-consolidated/compacted soils would be:
no geotextile (why are you using that with good soils?),
6-8" Course Agg. of #200 - 2", D50= 3/4" (PennDOT calls it "2A"),
2.5" BCBC (PennDOT calls it "ID-2"),
1.5" ID-2 Wear Course.
Engineering is the practice of the art of science - Steve
LHA sounds reasonable, but for a parking lot the traffic count is difficult to apply, since normally it is used for roadways with moving traffic that it is not spread all over the place, as in a parking lot.
The most severe condition for a pavement like this is slow, turning movements of heavy vehicles. Then too, the busses usually follow slowly behind each other in the same track when picking up kids. That is worse than the regular road situation. Well maybe parked semi-trailers with dollies on the pavement are worse.
So, go ahead and look at local practice for streets, such as collector streets, then beef it up a little to allow for your slow turning movements. Usually you can consider that one inch of asphaltic concrete is equal to about 3 inches of base course. So you see adding base is likely cheaper.
While it maybe is nice to theoretically call for a thicker pavement at the main traveled lanes, practically speaking a contractor would rather make everything the same thickness, perhaps over built back in corners, etc.
A prime requirement for long life of a pavement is to have a firm and unyielding earth subgrade before placing the base course. I usually spec that a fully load dump truck should not make a rut any deeper than 2 inches.
Oh yes, no way do you normally need fabric, unless that truck rut is something like 12 inches throughout the site. Then the design is a lot more complicated than we are discussing here.
The most severe condition for a pavement like this is slow, turning movements of heavy vehicles. Then too, the busses usually follow slowly behind each other in the same track when picking up kids. That is worse than the regular road situation. Well maybe parked semi-trailers with dollies on the pavement are worse.
So, go ahead and look at local practice for streets, such as collector streets, then beef it up a little to allow for your slow turning movements. Usually you can consider that one inch of asphaltic concrete is equal to about 3 inches of base course. So you see adding base is likely cheaper.
While it maybe is nice to theoretically call for a thicker pavement at the main traveled lanes, practically speaking a contractor would rather make everything the same thickness, perhaps over built back in corners, etc.
A prime requirement for long life of a pavement is to have a firm and unyielding earth subgrade before placing the base course. I usually spec that a fully load dump truck should not make a rut any deeper than 2 inches.
Oh yes, no way do you normally need fabric, unless that truck rut is something like 12 inches throughout the site. Then the design is a lot more complicated than we are discussing here.
The section proposed by LHA would work nicely but is a bit expensive due to the high asphalt thickness.
You don't say where you are geographically, but that can make a big difference. For instance, if you are not in a freeze-thaw area, asphalt thicknesses can be much less.
The structural number concept and numerical methods can certainly be applied to this application as LHA noted. No, you don't necessarily have good traffic information, but then on roadway design we rarely have "good" traffic information...it constantly changes. You do have predictable conditions (you know or can find out how many busses use the section per day and you know how many days of school there are in a year, and you know or can find the loaded/unloaded wheel loads for the busses. The small vehicle traffic is negligible, even if the number of cars is high....they don't have much effect on the pavement structural design.
As for the geotextile....I agree with the other comments...you don't need it unless you have unstable or yielding subsurface soils.
Lastly, make sure your specification calls for proper compaction AND stability of the structural layers. For instance, the subgrade should have a stability (measured by a CBR or similar stability testing...CBR of 25 to 30) and a compaction requirement (generally 98 percent of the maximum dry density as determined by the Modified Proctor method (AASHTO T180)). The base should also have a stability and compaction requirement (CBR>80 and compaction> 98%) and the asphalt should be compacted to at 96 to 98 percent of its laboratory density. Don't specify based on "rutting of a fully loaded dump truck"....that is not repeatable nor objective and has no standard to reference.
You don't say where you are geographically, but that can make a big difference. For instance, if you are not in a freeze-thaw area, asphalt thicknesses can be much less.
The structural number concept and numerical methods can certainly be applied to this application as LHA noted. No, you don't necessarily have good traffic information, but then on roadway design we rarely have "good" traffic information...it constantly changes. You do have predictable conditions (you know or can find out how many busses use the section per day and you know how many days of school there are in a year, and you know or can find the loaded/unloaded wheel loads for the busses. The small vehicle traffic is negligible, even if the number of cars is high....they don't have much effect on the pavement structural design.
As for the geotextile....I agree with the other comments...you don't need it unless you have unstable or yielding subsurface soils.
Lastly, make sure your specification calls for proper compaction AND stability of the structural layers. For instance, the subgrade should have a stability (measured by a CBR or similar stability testing...CBR of 25 to 30) and a compaction requirement (generally 98 percent of the maximum dry density as determined by the Modified Proctor method (AASHTO T180)). The base should also have a stability and compaction requirement (CBR>80 and compaction> 98%) and the asphalt should be compacted to at 96 to 98 percent of its laboratory density. Don't specify based on "rutting of a fully loaded dump truck"....that is not repeatable nor objective and has no standard to reference.
Hi Ron:
Sounds like your procedures for site preparation are rather elaborate, but they will create a nice completed job, that's for sure.
However, like you said, where is the job? I suspect it is not for my area with that preparation method.
In my area (Minnesota, Wisconin, Illinois, Iowa), we have some clays that naturally like to sit at 83 percent "compaction", plus or minus. Compact the heck out of them, with the associated drying and then wait a few years. Eventually they come back to 83 percent. I'd rather design for that weaker subgrade than spend money on a subgrade condition that is not permanent.
We usually design for subgrade CBR of 3, to allow for this and it does not require 98 percent compaction, with the associated cost and right weather conditions.
On structural number design, which came from the results of the Ottawa Road test,I often wonder how that can be applied nationwise when the testing was done in our area only. Another interesting thing about that work is they never came out with a final recommendation for design, only two Interim Guide(s) for Design. By the way, I do use the results of that work, when they can be shown to apply locally, with the suitable regional factor.
The conclusion I would offer to someone with no detailed knowledge of design methods, use what works locally, perhaps with a design by the local state asphalt association's recommendations.
Sounds like your procedures for site preparation are rather elaborate, but they will create a nice completed job, that's for sure.
However, like you said, where is the job? I suspect it is not for my area with that preparation method.
In my area (Minnesota, Wisconin, Illinois, Iowa), we have some clays that naturally like to sit at 83 percent "compaction", plus or minus. Compact the heck out of them, with the associated drying and then wait a few years. Eventually they come back to 83 percent. I'd rather design for that weaker subgrade than spend money on a subgrade condition that is not permanent.
We usually design for subgrade CBR of 3, to allow for this and it does not require 98 percent compaction, with the associated cost and right weather conditions.
On structural number design, which came from the results of the Ottawa Road test,I often wonder how that can be applied nationwise when the testing was done in our area only. Another interesting thing about that work is they never came out with a final recommendation for design, only two Interim Guide(s) for Design. By the way, I do use the results of that work, when they can be shown to apply locally, with the suitable regional factor.
The conclusion I would offer to someone with no detailed knowledge of design methods, use what works locally, perhaps with a design by the local state asphalt association's recommendations.
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